People are not always turning from crosswind. I think you make a good point if joining long downwind, but consistency is the main thing and it would be better if everyone was reminded of 'normal' procedures whatever they are. I was taught to call downwind at that specific point because it is easily identifiable. If two people call in short succession you'd better have a fast look around. However, I've heard people call downwind when they are a mile away. I've also had someone fly straight across me when I was halfway downwind, but I digress....